To provide communications between devices, systems, or components, the devices can be connected together with a cable. In some applications, it may be desirable for one device to determine whether the cable is connected at its other end to another device and whether the other device is powered up. However, in a situation where the connected devices are to maintain ground isolation, determining whether a device is connected to the cable and powered involves additional considerations than in a system where ground isolation is not important.
With reference to FIG. 1, a prior art system is shown that is configured to allow an upstream device 100 to determine whether a downstream device 105 is connected to a cable 110 and is powered. The cable 110 is shown with only two signal paths or conductors 115 and 120 that are used for the detection process although the cable 110 will include one or more other conductors to facilitate data communication between the devices. For example, the conductor 115 is used to carry power (e.g. a voltage) from a voltage source on the upstream device 100 to the downstream device 105. The voltage source is labeled as “+V upstream.” The conductor 120 is used to return a voltage signal to the upstream device 100 if the downstream device 105 is connected to the cable 110 and is powered. The returned voltage value can be read by the upstream device 100 as a value “+Presence” to determine whether the downstream device 105 is present and powered. The +Presence signal is connected through a pull-down resistor 125 to ground (GND) to stabilize the signal on conductor 120 in case the cable 110 is not connected. In this configuration, if the +Presence value is a logical high, this indicates that the downstream device 105 is connected and powered. If the +Presence value is a logic low, this means that the downstream device 105 is either not connected and/or not powered since the voltage from +V upstream is not conducted through the signal path 120.
Looking to the configuration of the downstream device 105, it is configured to return the voltage received on the signal path 115 back to the upstream device 100 along signal path 120. The voltage is returned along the signal path 120 only when the downstream device 105 is connected to the cable 110 and is powered. This is performed while maintaining ground isolation between the devices. In particular, the downstream device 105 includes an optical isolator 130 that is configured with a light-emitting diode 135 and a photo-sensor/phototransistor 140. The conductor 115 is connected to the collector of the phototransistor 140 and the conductor 120 is connected to the emitter of the phototransistor 140. The phototransistor 140 is configured such that when the diode 135 is in an off state, no light (or infrared) is detected by the phototransistor 140 causing the phototransistor to be in an off state. In the off state, the phototransistor 140 does not allow voltage/current to pass from the collector to the emitter, thus, no voltage/current is conducted to the signal path 120. The photodiode 135 is powered by a voltage source +V downstream from the downstream device 105 and is grounded to the downstream device 105. As such, ground isolation is maintained between the device 100 and the device 105. A current-limiting resistor 145 may be used to protect the light-emitting diode 135 from being overloaded.
With the system shown in FIG. 1, the upstream device 100 can detect whether the downstream device 105 is both connected to the cable 110 and is powered using the following scenario. Assuming that the downstream device 105 is connected to the cable 110 and is powered, power is supplied from the voltage source +V downstream that turns on the light-emitting diode 135, which in turn, causes the phototransistor 140 to turn on. In the on state, the phototransistor 140 allows voltage from the conductor 115 to pass through onto conductor 120 and be passed back to the upstream device 100. Voltage on the conductor 120 causes the value of +Presence to change from a logical low to a logical high, which indicates that the downstream device 105 is connected and powered. In either case where the downstream device 105 is not connected to the cable 110 or is not powered, the voltage from the conductor 115 does not reach the conductor 120 causing the value of the +Presence to be a logical low.
Although the system shown in FIG. 1 allows the upstream device 100 to detect whether the downstream device is connected and powered, it does not allow the downstream device 105 to determine whether the upstream device 100 is connected and powered. In some systems, it may be beneficial for both devices to determine if the other is connected and powered.